bcachefs: Assorted fixes for running on very small devices

It's now possible to create and use a filesystem on a 512k device with
4k buckets (though at that size we still waste almost half to internal
reserves)
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -374,6 +374,11 @@ static void bch2_rescale_bucket_io_times(struct bch_fs *c, int rw)
 	}
 }
 
+static inline u64 bucket_clock_freq(u64 capacity)
+{
+	return max(capacity >> 10, 2028ULL);
+}
+
 static void bch2_inc_clock_hand(struct io_timer *timer)
 {
 	struct bucket_clock *clock = container_of(timer,
@@ -412,7 +417,7 @@ static void bch2_inc_clock_hand(struct io_timer *timer)
 	 * RW mode (that will be 0 when we're RO, yet we can still service
 	 * reads)
 	 */
-	timer->expire += capacity >> 10;
+	timer->expire += bucket_clock_freq(capacity);
 
 	bch2_io_timer_add(&c->io_clock[clock->rw], timer);
 }
@@ -424,7 +429,7 @@ static void bch2_bucket_clock_init(struct bch_fs *c, int rw)
 	clock->hand		= 1;
 	clock->rw		= rw;
 	clock->rescale.fn	= bch2_inc_clock_hand;
-	clock->rescale.expire	= c->capacity >> 10;
+	clock->rescale.expire	= bucket_clock_freq(c->capacity);
 	mutex_init(&clock->lock);
 }
 
@@ -1011,8 +1016,6 @@ void bch2_recalc_capacity(struct bch_fs *c)
 		for (j = 0; j < RESERVE_NONE; j++)
 			dev_reserve += ca->free[j].size;
 
-		dev_reserve += ca->free_inc.size;
-
 		dev_reserve += 1;	/* btree write point */
 		dev_reserve += 1;	/* copygc write point */
 		dev_reserve += 1;	/* rebalance write point */

fs/bcachefs/alloc_background.h

@@ -6,7 +6,7 @@
 #include "alloc_types.h"
 #include "debug.h"
 
-#define ALLOC_SCAN_BATCH(ca)		((ca)->mi.nbuckets >> 9)
+#define ALLOC_SCAN_BATCH(ca)		max_t(size_t, 1, (ca)->mi.nbuckets >> 9)
 
 const char *bch2_alloc_invalid(const struct bch_fs *, struct bkey_s_c);
 int bch2_alloc_to_text(struct bch_fs *, char *, size_t, struct bkey_s_c);

fs/bcachefs/bcachefs.h

@@ -325,7 +325,7 @@ enum bch_time_stats {
 #define BTREE_RESERVE_MAX	(BTREE_MAX_DEPTH + (BTREE_MAX_DEPTH - 1))
 
 /* Size of the freelist we allocate btree nodes from: */
-#define BTREE_NODE_RESERVE	(BTREE_RESERVE_MAX * 4)
+#define BTREE_NODE_RESERVE	BTREE_RESERVE_MAX
 
 struct btree;
 

fs/bcachefs/bcachefs_format.h

@@ -909,6 +909,8 @@ struct bch_sb_field_journal {
 
 /* BCH_SB_FIELD_members: */
 
+#define BCH_MIN_NR_NBUCKETS	(1 << 6)
+
 struct bch_member {
 	__uuid_t		uuid;
 	__le64			nbuckets;	/* device size */
@@ -1391,7 +1393,7 @@ struct jset {
 LE32_BITMASK(JSET_CSUM_TYPE,	struct jset, flags, 0, 4);
 LE32_BITMASK(JSET_BIG_ENDIAN,	struct jset, flags, 4, 5);
 
-#define BCH_JOURNAL_BUCKETS_MIN		20
+#define BCH_JOURNAL_BUCKETS_MIN		8
 
 /* Btree: */
 

fs/bcachefs/buckets.c

@@ -887,9 +887,9 @@ int bch2_dev_buckets_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 	size_t btree_reserve	= DIV_ROUND_UP(BTREE_NODE_RESERVE,
 			     ca->mi.bucket_size / c->opts.btree_node_size);
 	/* XXX: these should be tunable */
-	size_t reserve_none	= max_t(size_t, 4, nbuckets >> 9);
-	size_t copygc_reserve	= max_t(size_t, 16, nbuckets >> 7);
-	size_t free_inc_nr	= max(max_t(size_t, 16, nbuckets >> 12),
+	size_t reserve_none	= max_t(size_t, 1, nbuckets >> 9);
+	size_t copygc_reserve	= max_t(size_t, 2, nbuckets >> 7);
+	size_t free_inc_nr	= max(max_t(size_t, 1, nbuckets >> 12),
 				      btree_reserve);
 	bool resize = ca->buckets != NULL,
 	     start_copygc = ca->copygc_thread != NULL;

fs/bcachefs/recovery.c

@@ -278,7 +278,7 @@ int bch2_fs_recovery(struct bch_fs *c)
 	return ret;
 err:
 fsck_err:
-	BUG_ON(!ret);
+	pr_err("Error in recovery: %s (%i)", err, ret);
 	goto out;
 }
 
@@ -381,6 +381,6 @@ int bch2_fs_initialize(struct bch_fs *c)
 
 	return 0;
 err:
-	BUG_ON(!ret);
+	pr_err("Error initializing new filesystem: %s (%i)", err, ret);
 	return ret;
 }

fs/bcachefs/super-io.c

@@ -811,7 +811,7 @@ static const char *bch2_sb_validate_members(struct bch_sb *sb,
 			return "Too many buckets";
 
 		if (le64_to_cpu(m->nbuckets) -
-		    le16_to_cpu(m->first_bucket) < 1 << 10)
+		    le16_to_cpu(m->first_bucket) < BCH_MIN_NR_NBUCKETS)
 			return "Not enough buckets";
 
 		if (le16_to_cpu(m->bucket_size) <